PORTLAND, Ore. (AP) – A defense attorney and friends suspect that a teenager accused of plotting mass killings in Portland was set up – groomed and talked into a plot to detonate what he thought were six 55-gallon drums of explosives in a van.

But prosecutors led by Attorney General Eric Holder say Mohamed Osman Mohamud plunged into a what turned out to be government sting, dismissing talk of backing out and also exhulting in the mayhem he expected as Portlanders gathered by the thousands last week for a Christmas tree-lighting celebration.

Mohamud "was told that children – children – were potentially going to be harmed," Holder said Monday as the 19-year-old native of Somalia appeared in court and his defenders attacked the government's case.

Outside the courtroom, a man who has played basketball with Mohamud said the teenager wouldn't have gotten involved in the plot without encouragement from the FBI.

"If you talk with someone enough, they'll be convinced they need to do something," said 20-year-old Muhahid El-Naser. He was among a small number of people gathered outside a federal court building about a five-block walk from what the government alleges was the target of the bomb plot last week, Pioneer Courthouse Square.

Inside the courthouse, public defender Stephen Sady was advancing similar arguments as he entered a not guilty plea on behalf of Mohamud.

Public defender Stephen Sady focused on the FBI's failed attempt to record a first conversation between Mohamud and an FBI undercover operative.

"In the cases involving potential entrapment, it's the initial meeting that matters," Sady said as he asked a judge to order the government to preserve recording equipment that was involved so that defense experts could examine it. Judge John Acosta did so.

In Washington, Holder defended the FBI sting, saying that once the undercover operation began, Mohamud "chose at every step to continue" with the plot.

Prosecutors say that agents let the plot string out to its end, with Mohamud feverishly dialing a cell phone number he thought would touch off the bomb, so that they could gather enough evidence to support the single charge he faces, attempted use of a weapon of mass destruction.

Holder also said the FBI was investigating a fire Sunday that destroyed part of an Islamic center in Corvallis, where Mohamud occasionally worshipped while attending Oregon State University.

Police believe the fire was a case of arson, and they increased patrols around mosques and other Islamic sites in Portland.

Also in Corvallis on Monday, authorities said a woman filed a sexual assault complaint against Mohamud after having sex with him following a fraternity party in 2009.

However, authorities said no charge was inked because tests did not turn up date rape drugs and two witnesses said it appeared to be consensual sex.